Atmospheric Hydrogen Peroxide Research Articles

Atmospheric H2O2 during haze episodes in a Chinese megacity: Concentration, source, and implication on sulfate production

Atmospheric hydrogen peroxide (H2O2), as an important oxidant, plays a key role in atmospheric chemistry. To reveal its characteristics in polluted areas, comprehensive observations were conducted in Zhengzhou, China from February 22 to March 4, 2019, including heavy pollution days (HP) and light pollution days (LP). High NO concentrations (18 ± 26 ppbv) were recorded in HP, preventing the recombination reaction of two HO2• radicals. Surprisingly, higher concentrations of H2O2 were observed in HP (1.5 ± 0.6 ppbv) than those in LP (1.2 ± 0.6 ppbv). In addition to low wind speed and relative humidity, the elevated H2O2 in HP could be mainly attributed to intensified particle-phase photoreactions and biomass burning. In terms of sulfate formation, transition-metal ions (TMI)-catalyzed oxidation emerged as the predominant oxidant pathway in both HP and LP. Note that the average H2O2 oxidation rate increased from 3.6 × 10−2 in LP to 1.1 × 10−1 μg m−3 h−1 in HP. Moreover, the oxidation by H2O2 might exceed that of TMI catalysis under specific conditions, emerging as the primary driver of sulfate formation.

Synergistic Effect of Hydrogen Peroxide and Cold Atmospheric Pressure Plasma-Jet for Microbial Disinfection

The efficiency of simultaneous treatment of the cold atmospheric pressure plasma jet (CAP) and hydrogen peroxide (H2O2) was investigated. A CAP with a thin and long plume was generated with Ar gas and applied to a common oral bacterium, Enterococcus faecalis (E. faecalis). The bactericidal efficiency was evaluated with the electron microscopy and the colony forming unit (CFU) assay. The underlying mechanisms were studied by measuring extracellular chemical changes in the water solution and by measuring biological responses such as the trans-membrane potential, the intracellular oxidative stress, and the membrane permeability. The combination of CAP with H2O2 could provide dramatic synergistic effects in bacterial disinfection through the enhanced membrane transportation of reactive species and the oxidation of intracellular molecules. Since the byproducts of both H2O2 and CAP are not significantly toxic, the synergistic bactericidal effects of their combination could be a good candidate to clinical applications.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere.

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Comparison of Fluorescent Techniques Using Two Enzymes Catalysed for Measurement of Atmospheric Peroxides

Atmospheric peroxides, especially hydrogen peroxide (H2O2), are essential oxidants. The peroxide concentration is closely related to the extent of OH radicals and the O3 cycle in the tropospheric atmospheric chemistry. However, only a few studies have investigated their atmospheric concentrations in China because of inadequacies in the measurement techniques or higher costs of analytical instruments. Therefore, it is essential to design a suitable analysis method of peroxides with higher sensitivity, lower detection limit, and low cost. In view of that, this study investigated the optimum analysis conditions of two H2O2 analytical techniques: the high-performance liquid chromatography (HPLC) with fluorescence detection using two-enzyme catalysis of horseradishperoxidase (HRP method) and Hemin (Hemin method). Furthermore, these two analysis methods were systematically compared in terms of detection limit, calibration curve, precision, accuracy, and applicability for the first time. The findings showed that the HRP method had a lower detection limit, higher sensitivity, and better applicability for detecting H2O2 and methyl hydroperoxide (MHP) than the Hemin method. Moreover, the HRP method is better suitable for H2O2 and MHP detection, which requires low detection limits and high sensitivity. Besides this, the Hemin method is inexpensive and is more suitable for detecting hydroxyl alkyl peroxides (C ≥ 3). The atmospheric concentrations (average) of H2O2 and MHP were 0.60 ± 0.37 ppb and 0.081 ± 0.039 ppb, respectively, as determined by the HRP method. Importantly, atmospheric peroxide concentrations were higher on sunny days than on cloudy days in Beijing in September 2016. H2O2 concentrations showed a diurnal variation with the lowest value in the morning and two peaks at 13:00–17:00. In contrast, MHP concentrations were lowest in the morning and highest after 17:00. Photochemical reactions were responsible for the production of H2O2 and MHP. The reactions of O3 and olefins emitted by motor vehicles also caused H2O2 concentration to increase during the evening rush hour.

Photochemical Aging of Atmospheric Fine Particles as a Potential Source for Gas-Phase Hydrogen Peroxide.

Atmospheric hydrogen peroxide (H2O2), as an important oxidant, plays a key role in atmospheric sulfate formation, affecting the global radiation budget and causing acid rain deposition. The disproportionation reactions of hydroperoxyl radicals (HO2) in both gas and aqueous phases have long been considered as dominant sources for atmospheric H2O2. However, these known sources cannot explain the significant formation of H2O2 in polluted areas under the conditions of high NO levels and low ambient relative humidity (RH). Here, we show that under relatively dry conditions during daytime, atmospheric fine particles directly produce abundant gas-phase H2O2. The formation of H2O2 is verified to be by a reaction between the particle surface -OH group and HO2 radicals formed by photooxidation of chromophoric dissolved organic matters (CDOMs), which is slightly influenced by the presence of high NO levels but remarkably accelerated by water vapor and O2. In contrast to aqueous-phase chemistry, transition metal ions (TMIs) are found to significantly suppress H2O2 formation from the atmospheric fine particles. The H2O2 formed from relatively dry particles can be directly involved in in situ SO2 oxidation, leading to sulfate formation. As CDOMs are ubiquitous in atmospheric fine particles, their daytime photochemistry is expected to play important roles in formation of H2O2 and sulfate worldwide.

Atmospheric Hydrogen Peroxide (H2O2) at the Foot and Summit of Mt. Tai: Variations, Sources and Sinks, and Implications for Ozone Formation Chemistry

AbstractHydrogen peroxide (H2O2) acts as a terminal sink for atmospheric HOx radicals (OH and HO2), playing a key role in tropospheric O3 formation. However, there are few field measurements of atmospheric H2O2 to assess its role in O3 formation, especially for the seriously polluted region of the North China Plain. In this study, H2O2 concentrations were measured at the foot of Mt. Tai from May to July 2018 and the summit of Mt. Tai from May to June 2019, with average values of 0.93 ± 1.01 and 2.05 ± 1.20 ppb, respectively. H2O2 exhibited a pronounced diurnal variation with a noon‐peak at the foot of Mt. Tai, which could be well reproduced by a gas‐phase box model with H2O2 dry deposition velocity of 3 cm s−1 included, indicating H2O2 was mainly photochemically produced. Modeling analysis showed H2O2 production at the foot was most sensitive to alkenes and aromatics, while the source and sink for H2O2 were dominated by HO2 recombination and dry deposition, respectively. Compared with the summer‐measurement in 2007, the remarkable elevation of H2O2 at the summit might be ascribed to volatile organic compounds (VOCs) increase and SO2 decline. Both H2O2‐O3 correlation and H2O2/NOz ratio suggested O3 formation at the foot of Mt. Tai was mainly VOC‐sensitive in the early morning and shifted to NOx‐sensitive thereafter. Therefore, reduction of VOCs emission especially for the reactive species of alkenes and aromatics in the morning as well as NOx emission around noontime will be effective for mitigating the serious O3 (as well as H2O2) pollution in Tai'an city.

4-(3-Methoxyphenyl)-5-(2-thienylmethyl)-2,4-dihydro-3H-1,2,4-triazole-3-selone: Synthesis, structural characteristics and reactions

The title compound 4-(3-methoxyphenyl)-3-(2-thienylmethyl)-4,5-dihydro-3H-1,2,4-triazole-3-selone 2 was synthesized by two-step procedure including condensation of 1-isoselenocyanato-3-methoxybenzene with 2-(2-thienyl)acetohydrazide to selenosemicarbazide 1 and its intramolecular cyclization. Oxidation of selone 2 by atmospheric oxygen or hydrogen peroxide furnished corresponding diselenide 3 or product of elimination of selenium 1,2,4-triazole 4. Alkylation of compound 2 by benzyl chloride leads to the formation of selenide 5. The synthesized compounds have been characterized by spectroscopic techniques and the structures of 3,4 and 5 were confirmed by X-ray diffraction techniques and the supramolecular contacts were analyzed by Hirshfeld surface analyses.

Atmospheric hydrogen peroxide at the foot and summit of Mt. Tai: variations, sources and sinks, and implications for ozone formation chemistry

Atmospheric hydrogen peroxide at the foot and summit of Mt. Tai: variations, sources and sinks, and implications for ozone formation chemistry

Assessment of p-aminophenol oxidation by simulating the process of hair dyeing and occurrence in hair salon wastewater and drinking water from treatment plant

This work reports the study of oxidation reaction of p-aminophenol (PAP) in ammoniacal medium in dissolved atmospheric oxygen and hydrogen peroxide, simulating the process of hair dyeing with permanent dyes. The products formed, which included semi-quinoneimine radical, quinoneimine, dimers, trimers and tetramers, were identified by mass spectrometry, infrared spectroscopy, UV–vis spectrophotometry, and nuclear magnetic resonance of hydrogen. The process was found to involve an autoxidation mechanism. The mutagenicity of the products was carried out by Salmonella Typhimurium YG1041 assay, and the results indicated no mutagenic properties. The presence of PAP and its oxidative products in samples of wastewater collected from hairdressing salon effluent (WW), raw river water (RRW), and water inlet and outlet of drinking water treatment plant (DWTP) was analyzed by HPLC-DAD. PAP was detected in the collected samples of WW, water samples from DWTP (before and after treatment), at concentrations of 2.1 ± 0.5 mg L−1, 1.9 ± 0.3 × 10-3 mg L−1 and 1.3 ± 0.2 × 10-3 mg L−1, respectively. The reaction products, including dimers, trimers and tetramers were identified only in the WW sample; this shows that both the precursor in the sample and its derivatives were released into the wastewater.

Combined Effect of Cold Atmospheric Plasma and Hydrogen Peroxide Treatment on Mature Listeria monocytogenes and Salmonella Typhimurium Biofilms.

Cold Atmospheric Plasma (CAP) is a promising novel method for biofilm inactivation as log-reduction values up to 4.0 log10 (CFU/cm2) have been reported. Nevertheless, as the efficacy of CAP itself is not sufficient for complete inactivation of mature biofilms, the hurdle technology could be applied in order to obtain higher combined efficacies. In this study, CAP treatment was combined with a mild hydrogen peroxide (H2O2) treatment for disinfection of 1 and 7 day(s) old Listeria monocytogenes and Salmonella Typhimurium biofilms. Three different treatment sequences were investigated in order to determine the most effective treatment sequence, i.e., (i) first CAP, then H2O2, (ii) first H2O2, then CAP, and (iii) a simultaneous treatment of CAP and H2O2. Removal of the biofilm, induction of sub-lethal injury, and H2O2 breakdown due to the presence of catalase within the biofilms were investigated in order to comment on their possible contribution to the combined inactivation efficacy. Results indicated that the preferred treatment sequence was dependent on the biofilm forming species, biofilm age, and applied H2O2 concentration [0.05 or 0.20% (v/v)]. At the lowest H2O2 concentration, the highest log-reductions were generally observed if the CAP treatment was preceded by the H2O2 treatment, while at the highest H2O2 concentration, the opposite sequence (first CAP, then H2O2) proved to be more effective. Induction of sub-lethal injury contributed to the combined bactericidal effect, while the presence of catalase within the biofilms resulted in an increased resistance. In addition, high log-reductions were partially the result of biofilm removal. The highest overall log-reductions [i.e., up to 5.42 ± 0.33 log10 (CFU/cm2)] were obtained at the highest H2O2 concentration if CAP treatment was followed by H2O2 treatment. As this resulted in almost complete inactivation of the L. monocytogenes and S. Typhimurium biofilms, the combined treatment of CAP and H2O2 proved to be a promising method for disinfection of abiotic surfaces.

High H2O2 Concentrations Observed during Haze Periods during the Winter in Beijing: Importance of H2O2 Oxidation in Sulfate Formation

Atmospheric hydrogen peroxide (H2O2) plays an important role in sulfate formation. To explore the contribution of the H2O2 oxidation pathway to atmospheric sulfate in winter in Beijing, three field campaigns of atmospheric H2O2 measurements were conducted at an urban site (Beijing) and a rural site (Wangdu) during the winter in 2016, 2017, and 2018. The H2O2 concentrations were usually around the detection limit (0.05 ppbv) during clean and severely polluted periods, whereas the highest H2O2 concentration of 0.90 ppbv was observed during moderately polluted periods. Obvious increases in the concentration of H2O2 could be observed after sunset at the urban site during each moderately polluted day, which was mainly attributed to transportation of H2O2-rich air from the rural areas in the south of Beijing. Coincident increases in the concentrations of H2O2 and PM2.5 were also observed during the day at high NO concentrations, implying that heterogeneous reactions might contribute to the formation of H2O2 und...

Impacts of heterogeneous reactions to atmospheric peroxides: Observations and budget analysis study

Atmospheric peroxides play important roles in atmospheric chemistry, acting as reactive oxidants and reservoirs of HOX and ROX radicals. Field measurements of atmospheric peroxides were conducted over urban Beijing from 2015 to 2016, including dust storm days, haze days and different seasons. We employed a box model based on RACM2 mechanism to conduct concentration simulation and budget analysis of hydrogen peroxide (H2O2) and peroxyacetic acid (PAA). In this study, heterogeneous reaction is found to be a significant sink for atmospheric H2O2 and PAA in urban Beijing. Here, we recommend a suitable uptake coefficient formula considering the water effect for model research of peroxides. It is found that H2O2 and PAA unexpectedly maintained considerable concentrations on haze days, even higher than that on non-haze days. This phenomenon is mainly ascribed to relatively high levels of volatile organic compounds and ozone on haze days. In addition, high levels of water vapor in pollution episode can promote not only the heterogeneous uptake to aerosol phase but also the production of H2O2. Atmospheric PAA formation is suggested to be sensitive to alkenes and NOX in urban Beijing. In particular, with the help of peroxides, sulfate formation rate from heterogeneous uptake could increase by ∼4 times on haze days, indicating the potential effect of peroxides on enhancement of aerosol oxidative property and secondary sulfate formation.

An ion-neutral model to investigate chemical ionization mass spectrometry analysis of atmospheric molecules – application to a mixed reagent ion system for hydroperoxides and organic acids

Abstract. An ion-neutral chemical kinetic model is described and used to simulate the negative ion chemistry occurring within a mixed-reagent ion chemical ionization mass spectrometer (CIMS). The model objective was the establishment of a theoretical basis to understand ambient pressure (variable sample flow and reagent ion carrier gas flow rates), water vapor, ozone and oxides of nitrogen effects on ion cluster sensitivities for hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), formic acid (HFo) and acetic acid (HAc). The model development started with established atmospheric ion chemistry mechanisms, thermodynamic data and reaction rate coefficients. The chemical mechanism was augmented with additional reactions and their reaction rate coefficients specific to the analytes. Some existing reaction rate coefficients were modified to enable the model to match laboratory and field campaign determinations of ion cluster sensitivities as functions of CIMS sample flow rate and ambient humidity. Relative trends in predicted and observed sensitivities are compared as instrument specific factors preclude a direct calculation of instrument sensitivity as a function of sample pressure and humidity. Predicted sensitivity trends and experimental sensitivity trends suggested the model captured the reagent ion and cluster chemistry and reproduced trends in ion cluster sensitivity with sample flow and humidity observed with a CIMS instrument developed for atmospheric peroxide measurements (PCIMSs). The model was further used to investigate the potential for isobaric compounds as interferences in the measurement of the above species. For ambient O3 mixing ratios more than 50 times those of H2O2, O3−(H2O) was predicted to be a significant isobaric interference to the measurement of H2O2 using O2−(H2O2) at m∕z 66. O3 and NO give rise to species and cluster ions, CO3−(H2O) and NO3−(H2O), respectively, which interfere in the measurement of CH3OOH using O2−(CH3OOH) at m∕z 80. The CO3−(H2O) interference assumed one of its O atoms was 18O and present in the cluster in proportion to its natural abundance. The model results indicated monitoring water vapor mixing ratio, m∕z 78 for CO3−(H2O) and m∕z 98 for isotopic CO3−(H2O)2 can be used to determine when CO3−(H2O) interference is significant. Similarly, monitoring water vapor mixing ratio, m∕z 62 for NO3− and m∕z 98 for NO3−(H2O)2 can be used to determine when NO3−(H2O) interference is significant.

Measurements of atmospheric hydrogen peroxide concentrations over Toyama Prefecture duringlate autumn usinga helicopter: On the comparison with the snow chemistry at a mountainous site

2016年11月30日の富山県射水市上空において，大気中の過酸化水素（H2O2）濃度の測定を，ヘリコプターを利用して実施した．二酸化硫黄（SO2）やホルムアルデヒド（HCHO）は地表付近で高濃度であったが，H2O2濃度は上空で高かった．観測されたH2O2濃度は，これまでに夏季に観測された濃度よりも非常に低かった．また，SO2濃度よりもH2O2 濃度が低く，SO2の液相酸化が抑制されているものと考えられる．同時期の立山・室堂平（標高2450m）で採取した新雪中のH2O2濃度と気象条件から計算された大気中のH2O2濃度と，ヘリコプターで観測した上空のH2O2濃度とを比較した結果，両者の値がほぼ一致し，山岳域での新雪中のH2O2濃度の測定から，上空大気中のH2O2濃度を推定できるものと考えられる．

Measurement of hydrogen peroxide and organic hydroperoxide concentrations during autumn in Beijing, China

Gaseous peroxides play important roles in atmospheric chemistry. To understand the pathways of the formation and removal of peroxides, atmospheric peroxide concentrations and their controlling factors were measured from 7:00 to 20:00 in September, October, and November 2013 at a heavily trafficked residential site in Beijing, China, with average concentrations of hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP) at 0.55ppb and 0.063ppb, respectively. H2O2 concentrations were higher in the afternoon and lower in the morning and evening, while MHP concentrations did not exhibit a regular diurnal pattern. Both H2O2 and MHP concentrations increased at dusk in most cases. Both peroxides displayed monthly variations with higher concentrations in September. These results suggested that photochemical activity was the main controlling factor on variations of H2O2 concentrations during the measurement period. Increasing concentrations of volatile organic compounds emitted by motor vehicles were important contributors to H2O2 and MHP enrichment. High levels of H2O2 and MHP concentrations which occurred during the measurement period probably resulted from the transport of a polluted air mass with high water vapor content passing over the Bohai Bay, China.

Observation of atmospheric peroxides during Wangdu Campaign 2014 at a rural site in the North China Plain

Abstract. Measurements of atmospheric peroxides were made during Wangdu Campaign 2014 at Wangdu, a rural site in the North China Plain (NCP) in summer 2014. The predominant peroxides were detected to be hydrogen peroxide (H2O2), methyl hydroperoxide (MHP) and peroxyacetic acid (PAA). The observed H2O2 reached up to 11.3 ppbv, which was the highest value compared with previous observations in China at summer time. A box model simulation based on the Master Chemical Mechanism and constrained by the simultaneous observations of physical parameters and chemical species was performed to explore the chemical budget of atmospheric peroxides. Photochemical oxidation of alkenes was found to be the major secondary formation pathway of atmospheric peroxides, while contributions from alkanes and aromatics were of minor importance. The comparison of modeled and measured peroxide concentrations revealed an underestimation during biomass burning events and an overestimation on haze days, which were ascribed to the direct production of peroxides from biomass burning and the heterogeneous uptake of peroxides by aerosols, respectively. The strengths of the primary emissions from biomass burning were on the same order of the known secondary production rates of atmospheric peroxides during the biomass burning events. The heterogeneous process on aerosol particles was suggested to be the predominant sink for atmospheric peroxides. The atmospheric lifetime of peroxides on haze days in summer in the NCP was about 2–3 h, which is in good agreement with the laboratory studies. Further comprehensive investigations are necessary to better understand the impact of biomass burning and heterogeneous uptake on the concentration of peroxides in the atmosphere.

Atmospheric hydrogen peroxide and Eoarchean iron formations

It is widely accepted that photosynthetic bacteria played a crucial role in Fe(II) oxidation and the precipitation of iron formations (IF) during the Late Archean-Early Paleoproterozoic (2.7-2.4 Ga). It is less clear whether microbes similarly caused the deposition of the oldest IF at ca. 3.8 Ga, which would imply photosynthesis having already evolved by that time. Abiological alternatives, such as the direct oxidation of dissolved Fe(II) by ultraviolet radiation may have occurred, but its importance has been discounted in environments where the injection of high concentrations of dissolved iron directly into the photic zone led to chemical precipitation reactions that overwhelmed photooxidation rates. However, an outstanding possibility remains with respect to photochemical reactions occurring in the atmosphere that might generate hydrogen peroxide (H2 O2 ), a recognized strong oxidant for ferrous iron. Here, we modeled the amount of H2 O2 that could be produced in an Eoarchean atmosphere using updated solar fluxes and plausible CO2 , O2 , and CH4 mixing ratios. Irrespective of the atmospheric simulations, the upper limit of H2 O2 rainout was calculated to be <10(6) molecules cm(-2) s(-1) . Using conservative Fe(III) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H2 O2 was insufficient by several orders of magnitude to account for IF deposition (requiring ~10(11) H2 O2 molecules cm(-2) s(-1) ). This finding further constrains the plausible Fe(II) oxidation mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe(II)-oxidizing micro-organisms the most likely mechanism responsible for Earth's oldest IF.

Atmospheric Peroxides in a Polluted Subtropical Environment: Seasonal Variation, Sources and Sinks, and Importance of Heterogeneous Processes

Hydrogen peroxide (H2O2) and organic peroxides play an important role in atmospheric chemistry, but knowledge of their abundances, sources, and sinks from heterogeneous processes remains incomplete. Here we report the measurement results obtained in four seasons during 2011-2012 at a suburban site and a background site in Hong Kong. Organic peroxides were found to be more abundant than H2O2, which is in contrast to most previous observations. Model calculations with a multiphase chemical mechanism suggest important contributions from heterogeneous processes (primarily transition metal ion [TMI]-HOx reactions) to the H2O2 budget, accounting for about one-third and more than half of total production rate and loss rate, respectively. In comparison, they contribute much less to organic peroxides. The fast removal of H2O2 by these heterogeneous reactions explains the observed high organic peroxide fractions. Sensitivity analysis reveals that the role of heterogeneous processes depends on the abundance of soluble metals in aerosol, serving as a net H2O2 source at low metal concentrations, but as a net sink with high metal loading. The findings of this study suggest the need to consider the chemical processes in the aerosol aqueous phase when examining the chemical budget of gas-phase H2O2.

Impacts of aerosols on the chemistry of atmospheric trace gases: a case study of peroxides and HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; radicals

Abstract. Field measurements of atmospheric peroxides were obtained during the summer on two consecutive years over urban Beijing, which highlighted the impacts of aerosols on the chemistry of peroxide compounds and hydroperoxyl radicals (HO2). The major peroxides were determined to be hydrogen peroxide (H2O2), methyl hydroperoxide (MHP), and peroxyacetic acid (PAA). A negative correlation was found between H2O2 and PAA in rainwater, providing evidence for a conversion between H2O2 and PAA in the aqueous phase. A standard gas phase chemistry model based on the NCAR Master Mechanism provided a good reproduction of the observed H2O2 profile on non-haze days but greatly overpredicted the H2O2 level on haze days. We attribute this overprediction to the reactive uptake of HO2 by the aerosols, since there was greatly enhanced aerosol loading and aerosol liquid water content on haze days. The discrepancy between the observed and modeled H2O2 can be diminished by adding to the model a newly proposed transition metal ion catalytic mechanism of HO2 in aqueous aerosols. This confirms the importance of the aerosol uptake of HO2 and the subsequent aqueous phase reactions in the reduction of H2O2. The closure of HO2 and H2O2 between the gas and aerosol phases suggests that the aerosols do not have a net reactive uptake of H2O2, because the conversion of HO2 to H2O2 on aerosols compensates for the H2O2 loss. Laboratory studies for the aerosol uptake of H2O2 in the presence of HO2 are urgently required to better understand the aerosol uptake of H2O2 in the real atmosphere.

Newly observed peroxides and the water effect on the formation and removal of hydroxyalkyl hydroperoxides in the ozonolysis of isoprene

Abstract. The ozonolysis of alkenes is considered to be an important source of atmospheric peroxides, which serve as oxidants, reservoirs of HOx radicals, and components of secondary organic aerosols (SOAs). Recent laboratory investigations of this reaction identified hydrogen peroxide (H2O2) and hydroxymethyl hydroperoxide (HMHP) in ozonolysis of isoprene. Although larger hydroxyalkyl hydroperoxides (HAHPs) were also expected, their presence is not currently supported by experimental evidence. In the present study, we investigated the formation of peroxides in the gas phase ozonolysis of isoprene at various relative humidities on a time scale of tens of seconds, using a quartz flow tube reactor coupled with the online detection of peroxides. We detected a variety of conventional peroxides, including H2O2, HMHP, methyl hydroperoxide, bis-hydroxymethyl hydroperoxide, and ethyl hydroperoxide, and interestingly found three unknown peroxides. The molar yields of the conventional peroxides fell within the range of values provided in the literature. The three unknown peroxides had a combined molar yield of ~ 30% at 5% relative humidity (RH), which was comparable with that of the conventional peroxides. Unlike H2O2 and HMHP, the molar yields of these three unknown peroxides were inversely related to the RH. On the basis of experimental kinetic and box model analysis, we tentatively assigned these unknown peroxides to C2−C4 HAHPs, which are produced by the reactions of different Criegee intermediates with water. Our study provides experimental evidence for the formation of large HAHPs in the ozonolysis of isoprene (one of the alkenes). These large HAHPs have a sufficiently long lifetime, estimated as tens of minutes, which allows them to become involved in atmospheric chemical processes, e.g., SOA formation and radical recycling.